A 42-year old man was admitted to our hospital for evaluation of pleural effusion in the right hemithorax. He had been treated for spastic paraplegia with dantrolene sodium for 28 months. The pleural fluid consisted of sterile exudates with a very high eosinophil count, and peripheral blood eosinophilia was noted. Thoracoscopy revealed no apparent abnormalities and a pleural biopsy specimen showed only non-specific inflammation. Two weeks after discontinuing dantrolene therapy, the pleural effusion disappeared. The toxicity of dantrolene in patients with pleural effusion must be taken into consideration when long-term dantrolene therapy is given.